1. Technical Field
The present specification relates to a nitride semiconductor light-emitting element and a method for fabricating such an element.
2. Description of the Related Art
A nitride semiconductor including nitrogen (N) as a Group V element is a prime candidate for a material to make a short-wave light-emitting element, because its bandgap is sufficiently wide. Among other things, gallium nitride-based compound semiconductors (which will be referred to herein as “GaN-based semiconductors”) have been researched and developed particularly extensively. As a result, blue-ray-emitting light-emitting diodes (LEDs), green-ray-emitting LEDs and semiconductor laser diodes formed of GaN-based semiconductors have already been used in actual products.
A GaN-based semiconductor has a wurtzite crystal structure. FIG. 1 schematically illustrates a unit cell of GaN. In an AlxGayInzN (where x+y+z=1, x≧0, y≧0, and z≧0) semiconductor crystal, some of the Ga atoms shown in FIG. 1 may be replaced with Al and/or In atoms.
FIG. 2 shows four vectors a1, a2, a3 and c, which are generally used to represent planes of a wurtzite crystal structure with four indices (i.e., hexagonal indices). The primitive vector a runs in the [0001] direction, which is called a “c-axis”. A plane that intersects with the c-axis at right angles is called either a “c-plane” or a “(0001) plane”. It should be noted that the “c-axis” and the “c-plane” are sometimes referred to herein as “C-axis” and “C-plane”. Thus, in the accompanying drawings, those axes and planes are identified by the capital letter to make them recognizable more easily.
In fabricating a semiconductor element using GaN-based semiconductors, a c-plane substrate, i.e., a substrate of which the principal surface is a (0001) plane, is generally used as a substrate on which GaN semiconductor crystals will be grown. In a c-plane, however, there is a slight shift in the c-axis direction between a Ga atom layer and a nitrogen atom layer, thus producing electrical polarization there. That is why the c-plane is also called a “polar plane”. As a result of the electrical polarization, a piezoelectric field is generated in the InGaN quantum well of the active layer in the c-axis direction. Once such a piezoelectric field has been generated in the active layer, some positional deviation occurs in the distributions of electrons and holes in the active layer due to the quantum confinement Stark effect of carriers. Consequently, the internal quantum efficiency decreases. Thus the threshold current is increased in a semiconductor laser diode. In an LED, the power dissipation is increased and the luminous efficiency is decreased. Meanwhile, as the density of injected carriers increases, the piezoelectric field is screened, thus varying the emission wavelength, too.
Thus, to overcome these problems, it has been proposed that a substrate, of which the principal surface is a non-polar plane such as a (10-10) plane, which is perpendicular to the [10-10] direction, be used. Such a (10-10) plane is called an “m-plane”. In this description, “-” attached on the left-hand side of a Miller-Bravais index in the parentheses means a “bar” (a negative direction index). As shown in FIG. 2, the m-plane is parallel to the c-axis (i.e., the primitive vector c) and intersects with the c-plane at right angles. On the m-plane, Ga atoms and nitrogen atoms are on the same atomic plane. For that reason, no electrical polarization will be produced perpendicularly to the m-plane. That is why if a semiconductor multilayer structure is formed perpendicularly to the m-plane, no piezoelectric field will be generated in the active layer, thus overcoming the problems described above.
In this case, the “m-plane” is a generic term that collectively refers to a family of planes including (10-10), (−1010), (1-100), (−1100), (01-10) and (0-110) planes. Also, in present specification, the “X-plane growth” means epitaxial growth that is produced perpendicularly to the X plane (where X=c or m) of a hexagonal wurtzite structure. As for the X-plane growth, the X plane will be sometimes referred to herein as a “growing plane”. Furthermore, a layer of semiconductor crystals that have been formed as a result of the X-plane growth will be sometimes referred to herein as an “X-plane semiconductor layer”.
Consequently, an LED that has been fabricated on such a substrate having a non-polar plane, for example, can achieve higher luminous efficiency than a conventional element that has been fabricated on a c-plane.
As can be seen, a GaN-based semiconductor element that has been grown on an m-plane substrate can achieve more significant effects, but has higher contact resistance, than what has been grown on a c-plane substrate. That is a problem.
Japanese Patent Publication No. 4568379 says that a GaN-based semiconductor light-emitting element, of which the principal surface is an m-plane, can have its contact resistance reduced by using a p-side electrode which is formed of an Mg layer that contacts with a p-type semiconductor region and an Ag layer that has been stacked on the Mg layer. Japanese Patent Publication No. 4568380 says that a GaN-based semiconductor light-emitting element, of which the principal surface is an m-plane, can have its contact resistance reduced by using an electrode made up of Zn and Ag. According to the disclosures of these Japanese Patent Publications, by going through a heat treatment, the element Ga diffuses from the p-type semiconductor region toward the p-side electrode and forms Ga vacancies acting as acceptors in the p-type semiconductor region, thus reducing the contact resistance.
On the other hand, Japanese Laid-Open Patent. Publication No. 2005-136415 discloses an electrode of Group III-V compound semiconductors which is characterized by including a first layer to be formed on a layer of a Group III-V nitride semiconductor and formed of a Zn-based material including a solute element in Zn and a second layer to be stacked on the first layer and formed of at least one substance selected from the group consisting of Au, Co, Pd, Pt, Ru, Rh, Ir, Ta, Cr, Mn, Mo, Tc, W, Re, Fe, Sc, Ti, Sn, Ge, Sb, Al, ITO, ZITO, ZIO, GIO, ZTO, FTO, AZO, GZO, In4Sn3O12 and Zn1-xMgxO (0≦x≦1).
Furthermore, Japanese Patent Publication No. 4417166 says that an Ag—Pd—Cu—Ge alloy with increased moisture resistance can be provided by replacing In included in an Ag—In—Cu—Ge alloy, which is a conventional decorative silver alloy, with Pd.
Furthermore, Japanese Laid-Open Patent Publication No. 2010-56423 discloses a p-side electrode 120 including an Ag alloy layer 120a, a Ti layer 120b, and an Au layer 120c which have been stacked in this order on a p-GaN contact layer 118. The Ag alloy layer 120a includes Ag as its principal component and Pd, Cu and Ge as additives. Since Ge has been added, interaction between Pb and Cu can be produced and good thermal and chemical stability can be achieved even at relatively low Pd and Cu concentrations. On top of that, by adding Ge to the Ag alloy layer 120a, a decrease in reflectance due to the addition of Pd and Cu can be minimized. The Ag alloy layer 120a is an alloy obtained by adding 1.0 mass % of Pd, 1.0 mass % of Cu, and 0.1 mass % of Ge to Ag.